There are thousands of individuals with absences of arms or hands in the United States alone and thousands of new amputees each year. Many of these amputees are fitted with electrically powered hand and arm prostheses. The utility of an electric hand prosthesis is greatly enhanced by the ability to rotate and flex the hand prosthesis. Once an object is grasped with a prosthetic hand, the object can be oriented for a desired task if the prosthesis can rotate and flex. A below the elbow amputee usually has four degrees of freedom available, which are humeral flexion/extension, abduction/adduction, rotation and elbow flexion. To orient an object into an arbitrary orientation, two or more degrees of freedom are essential. Also, simple tasks like holding a fork or a tray of food in the needed orientation are difficult to do without wrist flexion/extension.
Wrist rotation and wrist flexion allow the amputee to better orient grasped objects. Wrist rotation alone is not enough to provide this flexibility that most prosthesis users desire. However, currently available wrist flexion devices are heavy, long, and are not easily locked in position. Currently available wrist rotators and flexion devices can add several inches of length and several ounces of weight, making the prosthesis awkward and cumbersome. Some known rotators can add about 2.5 inches and 3.5 ounces to the weight of the prosthesis. Adding a wrist flexion device that is currently commercially available can add another 1 or 2 inches and 2 or 3 ounces. This means an addition of approximately 4 inches and 6 ounces to get both rotation and flexion. Four inches in the wrist area is too long for most amputees to use with ease, especially amputees who have retained most of their forearm. For this reason, few amputees utilize two or three degree-of-freedom wrists. This generally limits the usefulness of the prosthesis.